Intrusion Alarm Patents

Background

There are numerous sensors that can be used to detect intrusion into a
protected space. The sensor and it's processing circuitry can be
optimized for this application and that's the subject of many patents.

Counts geophone zero crossings
where there is normally a high number, but when an intruder is present
there are fewer crossings. Frequency only.
This is very similar to the noise operated squelch circuit. When
a signal is present it quiets the noise.
References:

3111657

AUTOMATIC
TRACKING FILTER

November 1963

Bagno

3331065

SIGNAL
PROCESSING CIRCUIT FOR INTRUSION ALARM SYSTEM

July 1967

McDonald

2730896

Circuit for Measuring Average
Intensity of Disturbances

January 1956

De Boisblanc

2903683

Sensivity Control of Apparatus
for Detecting Disturbances in an Enclosure

Doppler

There are a number of frequencies used for doppler systems. Door
openers commonly use 10.5 GHz. Commercial volume protection
alarms may use 915 Mhz. Military systems may use a frequency
where the height of a man is 1/2 wavelength or about 72 MHz.
There have been some attempts at using ultrasonic audio in doppler
systems but they have not worked very well.